1 //===- LoopAccessAnalysis.cpp - Loop Access Analysis Implementation --------==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // The implementation for the loop memory dependence that was originally
11 // developed for the loop vectorizer.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Analysis/LoopAccessAnalysis.h"
16 #include "llvm/Analysis/LoopInfo.h"
17 #include "llvm/Analysis/ScalarEvolutionExpander.h"
18 #include "llvm/Analysis/TargetLibraryInfo.h"
19 #include "llvm/Analysis/ValueTracking.h"
20 #include "llvm/IR/DiagnosticInfo.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/IRBuilder.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 #include "llvm/Analysis/VectorUtils.h"
28 #define DEBUG_TYPE "loop-accesses"
30 static cl::opt<unsigned, true>
31 VectorizationFactor("force-vector-width", cl::Hidden,
32 cl::desc("Sets the SIMD width. Zero is autoselect."),
33 cl::location(VectorizerParams::VectorizationFactor));
34 unsigned VectorizerParams::VectorizationFactor;
36 static cl::opt<unsigned, true>
37 VectorizationInterleave("force-vector-interleave", cl::Hidden,
38 cl::desc("Sets the vectorization interleave count. "
39 "Zero is autoselect."),
41 VectorizerParams::VectorizationInterleave));
42 unsigned VectorizerParams::VectorizationInterleave;
44 static cl::opt<unsigned, true> RuntimeMemoryCheckThreshold(
45 "runtime-memory-check-threshold", cl::Hidden,
46 cl::desc("When performing memory disambiguation checks at runtime do not "
47 "generate more than this number of comparisons (default = 8)."),
48 cl::location(VectorizerParams::RuntimeMemoryCheckThreshold), cl::init(8));
49 unsigned VectorizerParams::RuntimeMemoryCheckThreshold;
51 /// \brief The maximum iterations used to merge memory checks
52 static cl::opt<unsigned> MemoryCheckMergeThreshold(
53 "memory-check-merge-threshold", cl::Hidden,
54 cl::desc("Maximum number of comparisons done when trying to merge "
55 "runtime memory checks. (default = 100)"),
58 /// Maximum SIMD width.
59 const unsigned VectorizerParams::MaxVectorWidth = 64;
61 /// \brief We collect dependences up to this threshold.
62 static cl::opt<unsigned>
63 MaxDependences("max-dependences", cl::Hidden,
64 cl::desc("Maximum number of dependences collected by "
65 "loop-access analysis (default = 100)"),
68 bool VectorizerParams::isInterleaveForced() {
69 return ::VectorizationInterleave.getNumOccurrences() > 0;
72 void LoopAccessReport::emitAnalysis(const LoopAccessReport &Message,
73 const Function *TheFunction,
75 const char *PassName) {
76 DebugLoc DL = TheLoop->getStartLoc();
77 if (const Instruction *I = Message.getInstr())
78 DL = I->getDebugLoc();
79 emitOptimizationRemarkAnalysis(TheFunction->getContext(), PassName,
80 *TheFunction, DL, Message.str());
83 Value *llvm::stripIntegerCast(Value *V) {
84 if (CastInst *CI = dyn_cast<CastInst>(V))
85 if (CI->getOperand(0)->getType()->isIntegerTy())
86 return CI->getOperand(0);
90 const SCEV *llvm::replaceSymbolicStrideSCEV(ScalarEvolution *SE,
91 const ValueToValueMap &PtrToStride,
92 SCEVUnionPredicate &Preds,
93 Value *Ptr, Value *OrigPtr) {
94 const SCEV *OrigSCEV = SE->getSCEV(Ptr);
96 // If there is an entry in the map return the SCEV of the pointer with the
97 // symbolic stride replaced by one.
98 ValueToValueMap::const_iterator SI =
99 PtrToStride.find(OrigPtr ? OrigPtr : Ptr);
100 if (SI != PtrToStride.end()) {
101 Value *StrideVal = SI->second;
104 StrideVal = stripIntegerCast(StrideVal);
106 // Replace symbolic stride by one.
107 Value *One = ConstantInt::get(StrideVal->getType(), 1);
108 ValueToValueMap RewriteMap;
109 RewriteMap[StrideVal] = One;
111 const auto *U = cast<SCEVUnknown>(SE->getSCEV(StrideVal));
113 static_cast<const SCEVConstant *>(SE->getOne(StrideVal->getType()));
115 Preds.add(SE->getEqualPredicate(U, CT));
117 const SCEV *ByOne = SE->rewriteUsingPredicate(OrigSCEV, Preds);
118 DEBUG(dbgs() << "LAA: Replacing SCEV: " << *OrigSCEV << " by: " << *ByOne
123 // Otherwise, just return the SCEV of the original pointer.
127 void RuntimePointerChecking::insert(Loop *Lp, Value *Ptr, bool WritePtr,
128 unsigned DepSetId, unsigned ASId,
129 const ValueToValueMap &Strides,
130 SCEVUnionPredicate &Preds) {
131 // Get the stride replaced scev.
132 const SCEV *Sc = replaceSymbolicStrideSCEV(SE, Strides, Preds, Ptr);
133 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Sc);
134 assert(AR && "Invalid addrec expression");
135 const SCEV *Ex = SE->getBackedgeTakenCount(Lp);
137 const SCEV *ScStart = AR->getStart();
138 const SCEV *ScEnd = AR->evaluateAtIteration(Ex, *SE);
139 const SCEV *Step = AR->getStepRecurrence(*SE);
141 // For expressions with negative step, the upper bound is ScStart and the
142 // lower bound is ScEnd.
143 if (const SCEVConstant *CStep = dyn_cast<const SCEVConstant>(Step)) {
144 if (CStep->getValue()->isNegative())
145 std::swap(ScStart, ScEnd);
147 // Fallback case: the step is not constant, but the we can still
148 // get the upper and lower bounds of the interval by using min/max
150 ScStart = SE->getUMinExpr(ScStart, ScEnd);
151 ScEnd = SE->getUMaxExpr(AR->getStart(), ScEnd);
154 Pointers.emplace_back(Ptr, ScStart, ScEnd, WritePtr, DepSetId, ASId, Sc);
157 SmallVector<RuntimePointerChecking::PointerCheck, 4>
158 RuntimePointerChecking::generateChecks() const {
159 SmallVector<PointerCheck, 4> Checks;
161 for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
162 for (unsigned J = I + 1; J < CheckingGroups.size(); ++J) {
163 const RuntimePointerChecking::CheckingPtrGroup &CGI = CheckingGroups[I];
164 const RuntimePointerChecking::CheckingPtrGroup &CGJ = CheckingGroups[J];
166 if (needsChecking(CGI, CGJ))
167 Checks.push_back(std::make_pair(&CGI, &CGJ));
173 void RuntimePointerChecking::generateChecks(
174 MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
175 assert(Checks.empty() && "Checks is not empty");
176 groupChecks(DepCands, UseDependencies);
177 Checks = generateChecks();
180 bool RuntimePointerChecking::needsChecking(const CheckingPtrGroup &M,
181 const CheckingPtrGroup &N) const {
182 for (unsigned I = 0, EI = M.Members.size(); EI != I; ++I)
183 for (unsigned J = 0, EJ = N.Members.size(); EJ != J; ++J)
184 if (needsChecking(M.Members[I], N.Members[J]))
189 /// Compare \p I and \p J and return the minimum.
190 /// Return nullptr in case we couldn't find an answer.
191 static const SCEV *getMinFromExprs(const SCEV *I, const SCEV *J,
192 ScalarEvolution *SE) {
193 const SCEV *Diff = SE->getMinusSCEV(J, I);
194 const SCEVConstant *C = dyn_cast<const SCEVConstant>(Diff);
198 if (C->getValue()->isNegative())
203 bool RuntimePointerChecking::CheckingPtrGroup::addPointer(unsigned Index) {
204 const SCEV *Start = RtCheck.Pointers[Index].Start;
205 const SCEV *End = RtCheck.Pointers[Index].End;
207 // Compare the starts and ends with the known minimum and maximum
208 // of this set. We need to know how we compare against the min/max
209 // of the set in order to be able to emit memchecks.
210 const SCEV *Min0 = getMinFromExprs(Start, Low, RtCheck.SE);
214 const SCEV *Min1 = getMinFromExprs(End, High, RtCheck.SE);
218 // Update the low bound expression if we've found a new min value.
222 // Update the high bound expression if we've found a new max value.
226 Members.push_back(Index);
230 void RuntimePointerChecking::groupChecks(
231 MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
232 // We build the groups from dependency candidates equivalence classes
234 // - We know that pointers in the same equivalence class share
235 // the same underlying object and therefore there is a chance
236 // that we can compare pointers
237 // - We wouldn't be able to merge two pointers for which we need
238 // to emit a memcheck. The classes in DepCands are already
239 // conveniently built such that no two pointers in the same
240 // class need checking against each other.
242 // We use the following (greedy) algorithm to construct the groups
243 // For every pointer in the equivalence class:
244 // For each existing group:
245 // - if the difference between this pointer and the min/max bounds
246 // of the group is a constant, then make the pointer part of the
247 // group and update the min/max bounds of that group as required.
249 CheckingGroups.clear();
251 // If we need to check two pointers to the same underlying object
252 // with a non-constant difference, we shouldn't perform any pointer
253 // grouping with those pointers. This is because we can easily get
254 // into cases where the resulting check would return false, even when
255 // the accesses are safe.
257 // The following example shows this:
258 // for (i = 0; i < 1000; ++i)
259 // a[5000 + i * m] = a[i] + a[i + 9000]
261 // Here grouping gives a check of (5000, 5000 + 1000 * m) against
262 // (0, 10000) which is always false. However, if m is 1, there is no
263 // dependence. Not grouping the checks for a[i] and a[i + 9000] allows
264 // us to perform an accurate check in this case.
266 // The above case requires that we have an UnknownDependence between
267 // accesses to the same underlying object. This cannot happen unless
268 // ShouldRetryWithRuntimeCheck is set, and therefore UseDependencies
269 // is also false. In this case we will use the fallback path and create
270 // separate checking groups for all pointers.
272 // If we don't have the dependency partitions, construct a new
273 // checking pointer group for each pointer. This is also required
274 // for correctness, because in this case we can have checking between
275 // pointers to the same underlying object.
276 if (!UseDependencies) {
277 for (unsigned I = 0; I < Pointers.size(); ++I)
278 CheckingGroups.push_back(CheckingPtrGroup(I, *this));
282 unsigned TotalComparisons = 0;
284 DenseMap<Value *, unsigned> PositionMap;
285 for (unsigned Index = 0; Index < Pointers.size(); ++Index)
286 PositionMap[Pointers[Index].PointerValue] = Index;
288 // We need to keep track of what pointers we've already seen so we
289 // don't process them twice.
290 SmallSet<unsigned, 2> Seen;
292 // Go through all equivalence classes, get the the "pointer check groups"
293 // and add them to the overall solution. We use the order in which accesses
294 // appear in 'Pointers' to enforce determinism.
295 for (unsigned I = 0; I < Pointers.size(); ++I) {
296 // We've seen this pointer before, and therefore already processed
297 // its equivalence class.
301 MemoryDepChecker::MemAccessInfo Access(Pointers[I].PointerValue,
302 Pointers[I].IsWritePtr);
304 SmallVector<CheckingPtrGroup, 2> Groups;
305 auto LeaderI = DepCands.findValue(DepCands.getLeaderValue(Access));
307 // Because DepCands is constructed by visiting accesses in the order in
308 // which they appear in alias sets (which is deterministic) and the
309 // iteration order within an equivalence class member is only dependent on
310 // the order in which unions and insertions are performed on the
311 // equivalence class, the iteration order is deterministic.
312 for (auto MI = DepCands.member_begin(LeaderI), ME = DepCands.member_end();
314 unsigned Pointer = PositionMap[MI->getPointer()];
316 // Mark this pointer as seen.
317 Seen.insert(Pointer);
319 // Go through all the existing sets and see if we can find one
320 // which can include this pointer.
321 for (CheckingPtrGroup &Group : Groups) {
322 // Don't perform more than a certain amount of comparisons.
323 // This should limit the cost of grouping the pointers to something
324 // reasonable. If we do end up hitting this threshold, the algorithm
325 // will create separate groups for all remaining pointers.
326 if (TotalComparisons > MemoryCheckMergeThreshold)
331 if (Group.addPointer(Pointer)) {
338 // We couldn't add this pointer to any existing set or the threshold
339 // for the number of comparisons has been reached. Create a new group
340 // to hold the current pointer.
341 Groups.push_back(CheckingPtrGroup(Pointer, *this));
344 // We've computed the grouped checks for this partition.
345 // Save the results and continue with the next one.
346 std::copy(Groups.begin(), Groups.end(), std::back_inserter(CheckingGroups));
350 bool RuntimePointerChecking::arePointersInSamePartition(
351 const SmallVectorImpl<int> &PtrToPartition, unsigned PtrIdx1,
353 return (PtrToPartition[PtrIdx1] != -1 &&
354 PtrToPartition[PtrIdx1] == PtrToPartition[PtrIdx2]);
357 bool RuntimePointerChecking::needsChecking(unsigned I, unsigned J) const {
358 const PointerInfo &PointerI = Pointers[I];
359 const PointerInfo &PointerJ = Pointers[J];
361 // No need to check if two readonly pointers intersect.
362 if (!PointerI.IsWritePtr && !PointerJ.IsWritePtr)
365 // Only need to check pointers between two different dependency sets.
366 if (PointerI.DependencySetId == PointerJ.DependencySetId)
369 // Only need to check pointers in the same alias set.
370 if (PointerI.AliasSetId != PointerJ.AliasSetId)
376 void RuntimePointerChecking::printChecks(
377 raw_ostream &OS, const SmallVectorImpl<PointerCheck> &Checks,
378 unsigned Depth) const {
380 for (const auto &Check : Checks) {
381 const auto &First = Check.first->Members, &Second = Check.second->Members;
383 OS.indent(Depth) << "Check " << N++ << ":\n";
385 OS.indent(Depth + 2) << "Comparing group (" << Check.first << "):\n";
386 for (unsigned K = 0; K < First.size(); ++K)
387 OS.indent(Depth + 2) << *Pointers[First[K]].PointerValue << "\n";
389 OS.indent(Depth + 2) << "Against group (" << Check.second << "):\n";
390 for (unsigned K = 0; K < Second.size(); ++K)
391 OS.indent(Depth + 2) << *Pointers[Second[K]].PointerValue << "\n";
395 void RuntimePointerChecking::print(raw_ostream &OS, unsigned Depth) const {
397 OS.indent(Depth) << "Run-time memory checks:\n";
398 printChecks(OS, Checks, Depth);
400 OS.indent(Depth) << "Grouped accesses:\n";
401 for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
402 const auto &CG = CheckingGroups[I];
404 OS.indent(Depth + 2) << "Group " << &CG << ":\n";
405 OS.indent(Depth + 4) << "(Low: " << *CG.Low << " High: " << *CG.High
407 for (unsigned J = 0; J < CG.Members.size(); ++J) {
408 OS.indent(Depth + 6) << "Member: " << *Pointers[CG.Members[J]].Expr
415 /// \brief Analyses memory accesses in a loop.
417 /// Checks whether run time pointer checks are needed and builds sets for data
418 /// dependence checking.
419 class AccessAnalysis {
421 /// \brief Read or write access location.
422 typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
423 typedef SmallPtrSet<MemAccessInfo, 8> MemAccessInfoSet;
425 AccessAnalysis(const DataLayout &Dl, AliasAnalysis *AA, LoopInfo *LI,
426 MemoryDepChecker::DepCandidates &DA, SCEVUnionPredicate &Preds)
427 : DL(Dl), AST(*AA), LI(LI), DepCands(DA), IsRTCheckAnalysisNeeded(false),
430 /// \brief Register a load and whether it is only read from.
431 void addLoad(MemoryLocation &Loc, bool IsReadOnly) {
432 Value *Ptr = const_cast<Value*>(Loc.Ptr);
433 AST.add(Ptr, MemoryLocation::UnknownSize, Loc.AATags);
434 Accesses.insert(MemAccessInfo(Ptr, false));
436 ReadOnlyPtr.insert(Ptr);
439 /// \brief Register a store.
440 void addStore(MemoryLocation &Loc) {
441 Value *Ptr = const_cast<Value*>(Loc.Ptr);
442 AST.add(Ptr, MemoryLocation::UnknownSize, Loc.AATags);
443 Accesses.insert(MemAccessInfo(Ptr, true));
446 /// \brief Check whether we can check the pointers at runtime for
447 /// non-intersection.
449 /// Returns true if we need no check or if we do and we can generate them
450 /// (i.e. the pointers have computable bounds).
451 bool canCheckPtrAtRT(RuntimePointerChecking &RtCheck, ScalarEvolution *SE,
452 Loop *TheLoop, const ValueToValueMap &Strides,
453 bool ShouldCheckStride = false);
455 /// \brief Goes over all memory accesses, checks whether a RT check is needed
456 /// and builds sets of dependent accesses.
457 void buildDependenceSets() {
458 processMemAccesses();
461 /// \brief Initial processing of memory accesses determined that we need to
462 /// perform dependency checking.
464 /// Note that this can later be cleared if we retry memcheck analysis without
465 /// dependency checking (i.e. ShouldRetryWithRuntimeCheck).
466 bool isDependencyCheckNeeded() { return !CheckDeps.empty(); }
468 /// We decided that no dependence analysis would be used. Reset the state.
469 void resetDepChecks(MemoryDepChecker &DepChecker) {
471 DepChecker.clearDependences();
474 MemAccessInfoSet &getDependenciesToCheck() { return CheckDeps; }
477 typedef SetVector<MemAccessInfo> PtrAccessSet;
479 /// \brief Go over all memory access and check whether runtime pointer checks
480 /// are needed and build sets of dependency check candidates.
481 void processMemAccesses();
483 /// Set of all accesses.
484 PtrAccessSet Accesses;
486 const DataLayout &DL;
488 /// Set of accesses that need a further dependence check.
489 MemAccessInfoSet CheckDeps;
491 /// Set of pointers that are read only.
492 SmallPtrSet<Value*, 16> ReadOnlyPtr;
494 /// An alias set tracker to partition the access set by underlying object and
495 //intrinsic property (such as TBAA metadata).
500 /// Sets of potentially dependent accesses - members of one set share an
501 /// underlying pointer. The set "CheckDeps" identfies which sets really need a
502 /// dependence check.
503 MemoryDepChecker::DepCandidates &DepCands;
505 /// \brief Initial processing of memory accesses determined that we may need
506 /// to add memchecks. Perform the analysis to determine the necessary checks.
508 /// Note that, this is different from isDependencyCheckNeeded. When we retry
509 /// memcheck analysis without dependency checking
510 /// (i.e. ShouldRetryWithRuntimeCheck), isDependencyCheckNeeded is cleared
511 /// while this remains set if we have potentially dependent accesses.
512 bool IsRTCheckAnalysisNeeded;
514 /// The SCEV predicate containing all the SCEV-related assumptions.
515 SCEVUnionPredicate &Preds;
518 } // end anonymous namespace
520 /// \brief Check whether a pointer can participate in a runtime bounds check.
521 static bool hasComputableBounds(ScalarEvolution *SE,
522 const ValueToValueMap &Strides, Value *Ptr,
523 Loop *L, SCEVUnionPredicate &Preds) {
524 const SCEV *PtrScev = replaceSymbolicStrideSCEV(SE, Strides, Preds, Ptr);
525 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
529 return AR->isAffine();
532 bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
533 ScalarEvolution *SE, Loop *TheLoop,
534 const ValueToValueMap &StridesMap,
535 bool ShouldCheckStride) {
536 // Find pointers with computable bounds. We are going to use this information
537 // to place a runtime bound check.
540 bool NeedRTCheck = false;
541 if (!IsRTCheckAnalysisNeeded) return true;
543 bool IsDepCheckNeeded = isDependencyCheckNeeded();
545 // We assign a consecutive id to access from different alias sets.
546 // Accesses between different groups doesn't need to be checked.
548 for (auto &AS : AST) {
549 int NumReadPtrChecks = 0;
550 int NumWritePtrChecks = 0;
552 // We assign consecutive id to access from different dependence sets.
553 // Accesses within the same set don't need a runtime check.
554 unsigned RunningDepId = 1;
555 DenseMap<Value *, unsigned> DepSetId;
558 Value *Ptr = A.getValue();
559 bool IsWrite = Accesses.count(MemAccessInfo(Ptr, true));
560 MemAccessInfo Access(Ptr, IsWrite);
567 if (hasComputableBounds(SE, StridesMap, Ptr, TheLoop, Preds) &&
568 // When we run after a failing dependency check we have to make sure
569 // we don't have wrapping pointers.
570 (!ShouldCheckStride ||
571 isStridedPtr(SE, Ptr, TheLoop, StridesMap, Preds) == 1)) {
572 // The id of the dependence set.
575 if (IsDepCheckNeeded) {
576 Value *Leader = DepCands.getLeaderValue(Access).getPointer();
577 unsigned &LeaderId = DepSetId[Leader];
579 LeaderId = RunningDepId++;
582 // Each access has its own dependence set.
583 DepId = RunningDepId++;
585 RtCheck.insert(TheLoop, Ptr, IsWrite, DepId, ASId, StridesMap, Preds);
587 DEBUG(dbgs() << "LAA: Found a runtime check ptr:" << *Ptr << '\n');
589 DEBUG(dbgs() << "LAA: Can't find bounds for ptr:" << *Ptr << '\n');
594 // If we have at least two writes or one write and a read then we need to
595 // check them. But there is no need to checks if there is only one
596 // dependence set for this alias set.
598 // Note that this function computes CanDoRT and NeedRTCheck independently.
599 // For example CanDoRT=false, NeedRTCheck=false means that we have a pointer
600 // for which we couldn't find the bounds but we don't actually need to emit
601 // any checks so it does not matter.
602 if (!(IsDepCheckNeeded && CanDoRT && RunningDepId == 2))
603 NeedRTCheck |= (NumWritePtrChecks >= 2 || (NumReadPtrChecks >= 1 &&
604 NumWritePtrChecks >= 1));
609 // If the pointers that we would use for the bounds comparison have different
610 // address spaces, assume the values aren't directly comparable, so we can't
611 // use them for the runtime check. We also have to assume they could
612 // overlap. In the future there should be metadata for whether address spaces
614 unsigned NumPointers = RtCheck.Pointers.size();
615 for (unsigned i = 0; i < NumPointers; ++i) {
616 for (unsigned j = i + 1; j < NumPointers; ++j) {
617 // Only need to check pointers between two different dependency sets.
618 if (RtCheck.Pointers[i].DependencySetId ==
619 RtCheck.Pointers[j].DependencySetId)
621 // Only need to check pointers in the same alias set.
622 if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
625 Value *PtrI = RtCheck.Pointers[i].PointerValue;
626 Value *PtrJ = RtCheck.Pointers[j].PointerValue;
628 unsigned ASi = PtrI->getType()->getPointerAddressSpace();
629 unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
631 DEBUG(dbgs() << "LAA: Runtime check would require comparison between"
632 " different address spaces\n");
638 if (NeedRTCheck && CanDoRT)
639 RtCheck.generateChecks(DepCands, IsDepCheckNeeded);
641 DEBUG(dbgs() << "LAA: We need to do " << RtCheck.getNumberOfChecks()
642 << " pointer comparisons.\n");
644 RtCheck.Need = NeedRTCheck;
646 bool CanDoRTIfNeeded = !NeedRTCheck || CanDoRT;
647 if (!CanDoRTIfNeeded)
649 return CanDoRTIfNeeded;
652 void AccessAnalysis::processMemAccesses() {
653 // We process the set twice: first we process read-write pointers, last we
654 // process read-only pointers. This allows us to skip dependence tests for
655 // read-only pointers.
657 DEBUG(dbgs() << "LAA: Processing memory accesses...\n");
658 DEBUG(dbgs() << " AST: "; AST.dump());
659 DEBUG(dbgs() << "LAA: Accesses(" << Accesses.size() << "):\n");
661 for (auto A : Accesses)
662 dbgs() << "\t" << *A.getPointer() << " (" <<
663 (A.getInt() ? "write" : (ReadOnlyPtr.count(A.getPointer()) ?
664 "read-only" : "read")) << ")\n";
667 // The AliasSetTracker has nicely partitioned our pointers by metadata
668 // compatibility and potential for underlying-object overlap. As a result, we
669 // only need to check for potential pointer dependencies within each alias
671 for (auto &AS : AST) {
672 // Note that both the alias-set tracker and the alias sets themselves used
673 // linked lists internally and so the iteration order here is deterministic
674 // (matching the original instruction order within each set).
676 bool SetHasWrite = false;
678 // Map of pointers to last access encountered.
679 typedef DenseMap<Value*, MemAccessInfo> UnderlyingObjToAccessMap;
680 UnderlyingObjToAccessMap ObjToLastAccess;
682 // Set of access to check after all writes have been processed.
683 PtrAccessSet DeferredAccesses;
685 // Iterate over each alias set twice, once to process read/write pointers,
686 // and then to process read-only pointers.
687 for (int SetIteration = 0; SetIteration < 2; ++SetIteration) {
688 bool UseDeferred = SetIteration > 0;
689 PtrAccessSet &S = UseDeferred ? DeferredAccesses : Accesses;
692 Value *Ptr = AV.getValue();
694 // For a single memory access in AliasSetTracker, Accesses may contain
695 // both read and write, and they both need to be handled for CheckDeps.
697 if (AC.getPointer() != Ptr)
700 bool IsWrite = AC.getInt();
702 // If we're using the deferred access set, then it contains only
704 bool IsReadOnlyPtr = ReadOnlyPtr.count(Ptr) && !IsWrite;
705 if (UseDeferred && !IsReadOnlyPtr)
707 // Otherwise, the pointer must be in the PtrAccessSet, either as a
709 assert(((IsReadOnlyPtr && UseDeferred) || IsWrite ||
710 S.count(MemAccessInfo(Ptr, false))) &&
711 "Alias-set pointer not in the access set?");
713 MemAccessInfo Access(Ptr, IsWrite);
714 DepCands.insert(Access);
716 // Memorize read-only pointers for later processing and skip them in
717 // the first round (they need to be checked after we have seen all
718 // write pointers). Note: we also mark pointer that are not
719 // consecutive as "read-only" pointers (so that we check
720 // "a[b[i]] +="). Hence, we need the second check for "!IsWrite".
721 if (!UseDeferred && IsReadOnlyPtr) {
722 DeferredAccesses.insert(Access);
726 // If this is a write - check other reads and writes for conflicts. If
727 // this is a read only check other writes for conflicts (but only if
728 // there is no other write to the ptr - this is an optimization to
729 // catch "a[i] = a[i] + " without having to do a dependence check).
730 if ((IsWrite || IsReadOnlyPtr) && SetHasWrite) {
731 CheckDeps.insert(Access);
732 IsRTCheckAnalysisNeeded = true;
738 // Create sets of pointers connected by a shared alias set and
739 // underlying object.
740 typedef SmallVector<Value *, 16> ValueVector;
741 ValueVector TempObjects;
743 GetUnderlyingObjects(Ptr, TempObjects, DL, LI);
744 DEBUG(dbgs() << "Underlying objects for pointer " << *Ptr << "\n");
745 for (Value *UnderlyingObj : TempObjects) {
746 // nullptr never alias, don't join sets for pointer that have "null"
747 // in their UnderlyingObjects list.
748 if (isa<ConstantPointerNull>(UnderlyingObj))
751 UnderlyingObjToAccessMap::iterator Prev =
752 ObjToLastAccess.find(UnderlyingObj);
753 if (Prev != ObjToLastAccess.end())
754 DepCands.unionSets(Access, Prev->second);
756 ObjToLastAccess[UnderlyingObj] = Access;
757 DEBUG(dbgs() << " " << *UnderlyingObj << "\n");
765 static bool isInBoundsGep(Value *Ptr) {
766 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr))
767 return GEP->isInBounds();
771 /// \brief Return true if an AddRec pointer \p Ptr is unsigned non-wrapping,
772 /// i.e. monotonically increasing/decreasing.
773 static bool isNoWrapAddRec(Value *Ptr, const SCEVAddRecExpr *AR,
774 ScalarEvolution *SE, const Loop *L) {
775 // FIXME: This should probably only return true for NUW.
776 if (AR->getNoWrapFlags(SCEV::NoWrapMask))
779 // Scalar evolution does not propagate the non-wrapping flags to values that
780 // are derived from a non-wrapping induction variable because non-wrapping
781 // could be flow-sensitive.
783 // Look through the potentially overflowing instruction to try to prove
784 // non-wrapping for the *specific* value of Ptr.
786 // The arithmetic implied by an inbounds GEP can't overflow.
787 auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
788 if (!GEP || !GEP->isInBounds())
791 // Make sure there is only one non-const index and analyze that.
792 Value *NonConstIndex = nullptr;
793 for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
794 if (!isa<ConstantInt>(*Index)) {
797 NonConstIndex = *Index;
800 // The recurrence is on the pointer, ignore for now.
803 // The index in GEP is signed. It is non-wrapping if it's derived from a NSW
804 // AddRec using a NSW operation.
805 if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
806 if (OBO->hasNoSignedWrap() &&
807 // Assume constant for other the operand so that the AddRec can be
809 isa<ConstantInt>(OBO->getOperand(1))) {
810 auto *OpScev = SE->getSCEV(OBO->getOperand(0));
812 if (auto *OpAR = dyn_cast<SCEVAddRecExpr>(OpScev))
813 return OpAR->getLoop() == L && OpAR->getNoWrapFlags(SCEV::FlagNSW);
819 /// \brief Check whether the access through \p Ptr has a constant stride.
820 int llvm::isStridedPtr(ScalarEvolution *SE, Value *Ptr, const Loop *Lp,
821 const ValueToValueMap &StridesMap,
822 SCEVUnionPredicate &Preds) {
823 Type *Ty = Ptr->getType();
824 assert(Ty->isPointerTy() && "Unexpected non-ptr");
826 // Make sure that the pointer does not point to aggregate types.
827 auto *PtrTy = cast<PointerType>(Ty);
828 if (PtrTy->getElementType()->isAggregateType()) {
829 DEBUG(dbgs() << "LAA: Bad stride - Not a pointer to a scalar type"
834 const SCEV *PtrScev = replaceSymbolicStrideSCEV(SE, StridesMap, Preds, Ptr);
836 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
838 DEBUG(dbgs() << "LAA: Bad stride - Not an AddRecExpr pointer "
839 << *Ptr << " SCEV: " << *PtrScev << "\n");
843 // The accesss function must stride over the innermost loop.
844 if (Lp != AR->getLoop()) {
845 DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop " <<
846 *Ptr << " SCEV: " << *PtrScev << "\n");
849 // The address calculation must not wrap. Otherwise, a dependence could be
851 // An inbounds getelementptr that is a AddRec with a unit stride
852 // cannot wrap per definition. The unit stride requirement is checked later.
853 // An getelementptr without an inbounds attribute and unit stride would have
854 // to access the pointer value "0" which is undefined behavior in address
855 // space 0, therefore we can also vectorize this case.
856 bool IsInBoundsGEP = isInBoundsGep(Ptr);
857 bool IsNoWrapAddRec = isNoWrapAddRec(Ptr, AR, SE, Lp);
858 bool IsInAddressSpaceZero = PtrTy->getAddressSpace() == 0;
859 if (!IsNoWrapAddRec && !IsInBoundsGEP && !IsInAddressSpaceZero) {
860 DEBUG(dbgs() << "LAA: Bad stride - Pointer may wrap in the address space "
861 << *Ptr << " SCEV: " << *PtrScev << "\n");
865 // Check the step is constant.
866 const SCEV *Step = AR->getStepRecurrence(*SE);
868 // Calculate the pointer stride and check if it is constant.
869 const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
871 DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr <<
872 " SCEV: " << *PtrScev << "\n");
876 auto &DL = Lp->getHeader()->getModule()->getDataLayout();
877 int64_t Size = DL.getTypeAllocSize(PtrTy->getElementType());
878 const APInt &APStepVal = C->getValue()->getValue();
880 // Huge step value - give up.
881 if (APStepVal.getBitWidth() > 64)
884 int64_t StepVal = APStepVal.getSExtValue();
887 int64_t Stride = StepVal / Size;
888 int64_t Rem = StepVal % Size;
892 // If the SCEV could wrap but we have an inbounds gep with a unit stride we
893 // know we can't "wrap around the address space". In case of address space
894 // zero we know that this won't happen without triggering undefined behavior.
895 if (!IsNoWrapAddRec && (IsInBoundsGEP || IsInAddressSpaceZero) &&
896 Stride != 1 && Stride != -1)
902 bool MemoryDepChecker::Dependence::isSafeForVectorization(DepType Type) {
906 case BackwardVectorizable:
910 case ForwardButPreventsForwarding:
912 case BackwardVectorizableButPreventsForwarding:
915 llvm_unreachable("unexpected DepType!");
918 bool MemoryDepChecker::Dependence::isBackward() const {
922 case ForwardButPreventsForwarding:
926 case BackwardVectorizable:
928 case BackwardVectorizableButPreventsForwarding:
931 llvm_unreachable("unexpected DepType!");
934 bool MemoryDepChecker::Dependence::isPossiblyBackward() const {
935 return isBackward() || Type == Unknown;
938 bool MemoryDepChecker::Dependence::isForward() const {
941 case ForwardButPreventsForwarding:
946 case BackwardVectorizable:
948 case BackwardVectorizableButPreventsForwarding:
951 llvm_unreachable("unexpected DepType!");
954 bool MemoryDepChecker::couldPreventStoreLoadForward(unsigned Distance,
955 unsigned TypeByteSize) {
956 // If loads occur at a distance that is not a multiple of a feasible vector
957 // factor store-load forwarding does not take place.
958 // Positive dependences might cause troubles because vectorizing them might
959 // prevent store-load forwarding making vectorized code run a lot slower.
960 // a[i] = a[i-3] ^ a[i-8];
961 // The stores to a[i:i+1] don't align with the stores to a[i-3:i-2] and
962 // hence on your typical architecture store-load forwarding does not take
963 // place. Vectorizing in such cases does not make sense.
964 // Store-load forwarding distance.
965 const unsigned NumCyclesForStoreLoadThroughMemory = 8*TypeByteSize;
966 // Maximum vector factor.
967 unsigned MaxVFWithoutSLForwardIssues =
968 VectorizerParams::MaxVectorWidth * TypeByteSize;
969 if(MaxSafeDepDistBytes < MaxVFWithoutSLForwardIssues)
970 MaxVFWithoutSLForwardIssues = MaxSafeDepDistBytes;
972 for (unsigned vf = 2*TypeByteSize; vf <= MaxVFWithoutSLForwardIssues;
974 if (Distance % vf && Distance / vf < NumCyclesForStoreLoadThroughMemory) {
975 MaxVFWithoutSLForwardIssues = (vf >>=1);
980 if (MaxVFWithoutSLForwardIssues< 2*TypeByteSize) {
981 DEBUG(dbgs() << "LAA: Distance " << Distance <<
982 " that could cause a store-load forwarding conflict\n");
986 if (MaxVFWithoutSLForwardIssues < MaxSafeDepDistBytes &&
987 MaxVFWithoutSLForwardIssues !=
988 VectorizerParams::MaxVectorWidth * TypeByteSize)
989 MaxSafeDepDistBytes = MaxVFWithoutSLForwardIssues;
993 /// \brief Check the dependence for two accesses with the same stride \p Stride.
994 /// \p Distance is the positive distance and \p TypeByteSize is type size in
997 /// \returns true if they are independent.
998 static bool areStridedAccessesIndependent(unsigned Distance, unsigned Stride,
999 unsigned TypeByteSize) {
1000 assert(Stride > 1 && "The stride must be greater than 1");
1001 assert(TypeByteSize > 0 && "The type size in byte must be non-zero");
1002 assert(Distance > 0 && "The distance must be non-zero");
1004 // Skip if the distance is not multiple of type byte size.
1005 if (Distance % TypeByteSize)
1008 unsigned ScaledDist = Distance / TypeByteSize;
1010 // No dependence if the scaled distance is not multiple of the stride.
1012 // for (i = 0; i < 1024 ; i += 4)
1013 // A[i+2] = A[i] + 1;
1015 // Two accesses in memory (scaled distance is 2, stride is 4):
1016 // | A[0] | | | | A[4] | | | |
1017 // | | | A[2] | | | | A[6] | |
1020 // for (i = 0; i < 1024 ; i += 3)
1021 // A[i+4] = A[i] + 1;
1023 // Two accesses in memory (scaled distance is 4, stride is 3):
1024 // | A[0] | | | A[3] | | | A[6] | | |
1025 // | | | | | A[4] | | | A[7] | |
1026 return ScaledDist % Stride;
1029 MemoryDepChecker::Dependence::DepType
1030 MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
1031 const MemAccessInfo &B, unsigned BIdx,
1032 const ValueToValueMap &Strides) {
1033 assert (AIdx < BIdx && "Must pass arguments in program order");
1035 Value *APtr = A.getPointer();
1036 Value *BPtr = B.getPointer();
1037 bool AIsWrite = A.getInt();
1038 bool BIsWrite = B.getInt();
1040 // Two reads are independent.
1041 if (!AIsWrite && !BIsWrite)
1042 return Dependence::NoDep;
1044 // We cannot check pointers in different address spaces.
1045 if (APtr->getType()->getPointerAddressSpace() !=
1046 BPtr->getType()->getPointerAddressSpace())
1047 return Dependence::Unknown;
1049 const SCEV *AScev = replaceSymbolicStrideSCEV(SE, Strides, Preds, APtr);
1050 const SCEV *BScev = replaceSymbolicStrideSCEV(SE, Strides, Preds, BPtr);
1052 int StrideAPtr = isStridedPtr(SE, APtr, InnermostLoop, Strides, Preds);
1053 int StrideBPtr = isStridedPtr(SE, BPtr, InnermostLoop, Strides, Preds);
1055 const SCEV *Src = AScev;
1056 const SCEV *Sink = BScev;
1058 // If the induction step is negative we have to invert source and sink of the
1060 if (StrideAPtr < 0) {
1063 std::swap(APtr, BPtr);
1064 std::swap(Src, Sink);
1065 std::swap(AIsWrite, BIsWrite);
1066 std::swap(AIdx, BIdx);
1067 std::swap(StrideAPtr, StrideBPtr);
1070 const SCEV *Dist = SE->getMinusSCEV(Sink, Src);
1072 DEBUG(dbgs() << "LAA: Src Scev: " << *Src << "Sink Scev: " << *Sink
1073 << "(Induction step: " << StrideAPtr << ")\n");
1074 DEBUG(dbgs() << "LAA: Distance for " << *InstMap[AIdx] << " to "
1075 << *InstMap[BIdx] << ": " << *Dist << "\n");
1077 // Need accesses with constant stride. We don't want to vectorize
1078 // "A[B[i]] += ..." and similar code or pointer arithmetic that could wrap in
1079 // the address space.
1080 if (!StrideAPtr || !StrideBPtr || StrideAPtr != StrideBPtr){
1081 DEBUG(dbgs() << "Pointer access with non-constant stride\n");
1082 return Dependence::Unknown;
1085 const SCEVConstant *C = dyn_cast<SCEVConstant>(Dist);
1087 DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
1088 ShouldRetryWithRuntimeCheck = true;
1089 return Dependence::Unknown;
1092 Type *ATy = APtr->getType()->getPointerElementType();
1093 Type *BTy = BPtr->getType()->getPointerElementType();
1094 auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
1095 unsigned TypeByteSize = DL.getTypeAllocSize(ATy);
1097 // Negative distances are not plausible dependencies.
1098 const APInt &Val = C->getValue()->getValue();
1099 if (Val.isNegative()) {
1100 bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
1101 if (IsTrueDataDependence &&
1102 (couldPreventStoreLoadForward(Val.abs().getZExtValue(), TypeByteSize) ||
1104 return Dependence::ForwardButPreventsForwarding;
1106 DEBUG(dbgs() << "LAA: Dependence is negative: NoDep\n");
1107 return Dependence::Forward;
1110 // Write to the same location with the same size.
1111 // Could be improved to assert type sizes are the same (i32 == float, etc).
1114 return Dependence::Forward;
1115 DEBUG(dbgs() << "LAA: Zero dependence difference but different types\n");
1116 return Dependence::Unknown;
1119 assert(Val.isStrictlyPositive() && "Expect a positive value");
1123 "LAA: ReadWrite-Write positive dependency with different types\n");
1124 return Dependence::Unknown;
1127 unsigned Distance = (unsigned) Val.getZExtValue();
1129 unsigned Stride = std::abs(StrideAPtr);
1131 areStridedAccessesIndependent(Distance, Stride, TypeByteSize)) {
1132 DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
1133 return Dependence::NoDep;
1136 // Bail out early if passed-in parameters make vectorization not feasible.
1137 unsigned ForcedFactor = (VectorizerParams::VectorizationFactor ?
1138 VectorizerParams::VectorizationFactor : 1);
1139 unsigned ForcedUnroll = (VectorizerParams::VectorizationInterleave ?
1140 VectorizerParams::VectorizationInterleave : 1);
1141 // The minimum number of iterations for a vectorized/unrolled version.
1142 unsigned MinNumIter = std::max(ForcedFactor * ForcedUnroll, 2U);
1144 // It's not vectorizable if the distance is smaller than the minimum distance
1145 // needed for a vectroized/unrolled version. Vectorizing one iteration in
1146 // front needs TypeByteSize * Stride. Vectorizing the last iteration needs
1147 // TypeByteSize (No need to plus the last gap distance).
1149 // E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
1151 // int *B = (int *)((char *)A + 14);
1152 // for (i = 0 ; i < 1024 ; i += 2)
1156 // Two accesses in memory (stride is 2):
1157 // | A[0] | | A[2] | | A[4] | | A[6] | |
1158 // | B[0] | | B[2] | | B[4] |
1160 // Distance needs for vectorizing iterations except the last iteration:
1161 // 4 * 2 * (MinNumIter - 1). Distance needs for the last iteration: 4.
1162 // So the minimum distance needed is: 4 * 2 * (MinNumIter - 1) + 4.
1164 // If MinNumIter is 2, it is vectorizable as the minimum distance needed is
1165 // 12, which is less than distance.
1167 // If MinNumIter is 4 (Say if a user forces the vectorization factor to be 4),
1168 // the minimum distance needed is 28, which is greater than distance. It is
1169 // not safe to do vectorization.
1170 unsigned MinDistanceNeeded =
1171 TypeByteSize * Stride * (MinNumIter - 1) + TypeByteSize;
1172 if (MinDistanceNeeded > Distance) {
1173 DEBUG(dbgs() << "LAA: Failure because of positive distance " << Distance
1175 return Dependence::Backward;
1178 // Unsafe if the minimum distance needed is greater than max safe distance.
1179 if (MinDistanceNeeded > MaxSafeDepDistBytes) {
1180 DEBUG(dbgs() << "LAA: Failure because it needs at least "
1181 << MinDistanceNeeded << " size in bytes");
1182 return Dependence::Backward;
1185 // Positive distance bigger than max vectorization factor.
1186 // FIXME: Should use max factor instead of max distance in bytes, which could
1187 // not handle different types.
1188 // E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
1189 // void foo (int *A, char *B) {
1190 // for (unsigned i = 0; i < 1024; i++) {
1191 // A[i+2] = A[i] + 1;
1192 // B[i+2] = B[i] + 1;
1196 // This case is currently unsafe according to the max safe distance. If we
1197 // analyze the two accesses on array B, the max safe dependence distance
1198 // is 2. Then we analyze the accesses on array A, the minimum distance needed
1199 // is 8, which is less than 2 and forbidden vectorization, But actually
1200 // both A and B could be vectorized by 2 iterations.
1201 MaxSafeDepDistBytes =
1202 Distance < MaxSafeDepDistBytes ? Distance : MaxSafeDepDistBytes;
1204 bool IsTrueDataDependence = (!AIsWrite && BIsWrite);
1205 if (IsTrueDataDependence &&
1206 couldPreventStoreLoadForward(Distance, TypeByteSize))
1207 return Dependence::BackwardVectorizableButPreventsForwarding;
1209 DEBUG(dbgs() << "LAA: Positive distance " << Val.getSExtValue()
1210 << " with max VF = "
1211 << MaxSafeDepDistBytes / (TypeByteSize * Stride) << '\n');
1213 return Dependence::BackwardVectorizable;
1216 bool MemoryDepChecker::areDepsSafe(DepCandidates &AccessSets,
1217 MemAccessInfoSet &CheckDeps,
1218 const ValueToValueMap &Strides) {
1220 MaxSafeDepDistBytes = -1U;
1221 while (!CheckDeps.empty()) {
1222 MemAccessInfo CurAccess = *CheckDeps.begin();
1224 // Get the relevant memory access set.
1225 EquivalenceClasses<MemAccessInfo>::iterator I =
1226 AccessSets.findValue(AccessSets.getLeaderValue(CurAccess));
1228 // Check accesses within this set.
1229 EquivalenceClasses<MemAccessInfo>::member_iterator AI, AE;
1230 AI = AccessSets.member_begin(I), AE = AccessSets.member_end();
1232 // Check every access pair.
1234 CheckDeps.erase(*AI);
1235 EquivalenceClasses<MemAccessInfo>::member_iterator OI = std::next(AI);
1237 // Check every accessing instruction pair in program order.
1238 for (std::vector<unsigned>::iterator I1 = Accesses[*AI].begin(),
1239 I1E = Accesses[*AI].end(); I1 != I1E; ++I1)
1240 for (std::vector<unsigned>::iterator I2 = Accesses[*OI].begin(),
1241 I2E = Accesses[*OI].end(); I2 != I2E; ++I2) {
1242 auto A = std::make_pair(&*AI, *I1);
1243 auto B = std::make_pair(&*OI, *I2);
1249 Dependence::DepType Type =
1250 isDependent(*A.first, A.second, *B.first, B.second, Strides);
1251 SafeForVectorization &= Dependence::isSafeForVectorization(Type);
1253 // Gather dependences unless we accumulated MaxDependences
1254 // dependences. In that case return as soon as we find the first
1255 // unsafe dependence. This puts a limit on this quadratic
1257 if (RecordDependences) {
1258 if (Type != Dependence::NoDep)
1259 Dependences.push_back(Dependence(A.second, B.second, Type));
1261 if (Dependences.size() >= MaxDependences) {
1262 RecordDependences = false;
1263 Dependences.clear();
1264 DEBUG(dbgs() << "Too many dependences, stopped recording\n");
1267 if (!RecordDependences && !SafeForVectorization)
1276 DEBUG(dbgs() << "Total Dependences: " << Dependences.size() << "\n");
1277 return SafeForVectorization;
1280 SmallVector<Instruction *, 4>
1281 MemoryDepChecker::getInstructionsForAccess(Value *Ptr, bool isWrite) const {
1282 MemAccessInfo Access(Ptr, isWrite);
1283 auto &IndexVector = Accesses.find(Access)->second;
1285 SmallVector<Instruction *, 4> Insts;
1286 std::transform(IndexVector.begin(), IndexVector.end(),
1287 std::back_inserter(Insts),
1288 [&](unsigned Idx) { return this->InstMap[Idx]; });
1292 const char *MemoryDepChecker::Dependence::DepName[] = {
1293 "NoDep", "Unknown", "Forward", "ForwardButPreventsForwarding", "Backward",
1294 "BackwardVectorizable", "BackwardVectorizableButPreventsForwarding"};
1296 void MemoryDepChecker::Dependence::print(
1297 raw_ostream &OS, unsigned Depth,
1298 const SmallVectorImpl<Instruction *> &Instrs) const {
1299 OS.indent(Depth) << DepName[Type] << ":\n";
1300 OS.indent(Depth + 2) << *Instrs[Source] << " -> \n";
1301 OS.indent(Depth + 2) << *Instrs[Destination] << "\n";
1304 bool LoopAccessInfo::canAnalyzeLoop() {
1305 // We need to have a loop header.
1306 DEBUG(dbgs() << "LAA: Found a loop: " <<
1307 TheLoop->getHeader()->getName() << '\n');
1309 // We can only analyze innermost loops.
1310 if (!TheLoop->empty()) {
1311 DEBUG(dbgs() << "LAA: loop is not the innermost loop\n");
1312 emitAnalysis(LoopAccessReport() << "loop is not the innermost loop");
1316 // We must have a single backedge.
1317 if (TheLoop->getNumBackEdges() != 1) {
1318 DEBUG(dbgs() << "LAA: loop control flow is not understood by analyzer\n");
1320 LoopAccessReport() <<
1321 "loop control flow is not understood by analyzer");
1325 // We must have a single exiting block.
1326 if (!TheLoop->getExitingBlock()) {
1327 DEBUG(dbgs() << "LAA: loop control flow is not understood by analyzer\n");
1329 LoopAccessReport() <<
1330 "loop control flow is not understood by analyzer");
1334 // We only handle bottom-tested loops, i.e. loop in which the condition is
1335 // checked at the end of each iteration. With that we can assume that all
1336 // instructions in the loop are executed the same number of times.
1337 if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
1338 DEBUG(dbgs() << "LAA: loop control flow is not understood by analyzer\n");
1340 LoopAccessReport() <<
1341 "loop control flow is not understood by analyzer");
1345 // ScalarEvolution needs to be able to find the exit count.
1346 const SCEV *ExitCount = SE->getBackedgeTakenCount(TheLoop);
1347 if (ExitCount == SE->getCouldNotCompute()) {
1348 emitAnalysis(LoopAccessReport() <<
1349 "could not determine number of loop iterations");
1350 DEBUG(dbgs() << "LAA: SCEV could not compute the loop exit count.\n");
1357 void LoopAccessInfo::analyzeLoop(const ValueToValueMap &Strides) {
1359 typedef SmallVector<Value*, 16> ValueVector;
1360 typedef SmallPtrSet<Value*, 16> ValueSet;
1362 // Holds the Load and Store *instructions*.
1366 // Holds all the different accesses in the loop.
1367 unsigned NumReads = 0;
1368 unsigned NumReadWrites = 0;
1370 PtrRtChecking.Pointers.clear();
1371 PtrRtChecking.Need = false;
1373 const bool IsAnnotatedParallel = TheLoop->isAnnotatedParallel();
1376 for (Loop::block_iterator bb = TheLoop->block_begin(),
1377 be = TheLoop->block_end(); bb != be; ++bb) {
1379 // Scan the BB and collect legal loads and stores.
1380 for (BasicBlock::iterator it = (*bb)->begin(), e = (*bb)->end(); it != e;
1383 // If this is a load, save it. If this instruction can read from memory
1384 // but is not a load, then we quit. Notice that we don't handle function
1385 // calls that read or write.
1386 if (it->mayReadFromMemory()) {
1387 // Many math library functions read the rounding mode. We will only
1388 // vectorize a loop if it contains known function calls that don't set
1389 // the flag. Therefore, it is safe to ignore this read from memory.
1390 CallInst *Call = dyn_cast<CallInst>(it);
1391 if (Call && getIntrinsicIDForCall(Call, TLI))
1394 // If the function has an explicit vectorized counterpart, we can safely
1395 // assume that it can be vectorized.
1396 if (Call && !Call->isNoBuiltin() && Call->getCalledFunction() &&
1397 TLI->isFunctionVectorizable(Call->getCalledFunction()->getName()))
1400 LoadInst *Ld = dyn_cast<LoadInst>(it);
1401 if (!Ld || (!Ld->isSimple() && !IsAnnotatedParallel)) {
1402 emitAnalysis(LoopAccessReport(Ld)
1403 << "read with atomic ordering or volatile read");
1404 DEBUG(dbgs() << "LAA: Found a non-simple load.\n");
1409 Loads.push_back(Ld);
1410 DepChecker.addAccess(Ld);
1414 // Save 'store' instructions. Abort if other instructions write to memory.
1415 if (it->mayWriteToMemory()) {
1416 StoreInst *St = dyn_cast<StoreInst>(it);
1418 emitAnalysis(LoopAccessReport(&*it) <<
1419 "instruction cannot be vectorized");
1423 if (!St->isSimple() && !IsAnnotatedParallel) {
1424 emitAnalysis(LoopAccessReport(St)
1425 << "write with atomic ordering or volatile write");
1426 DEBUG(dbgs() << "LAA: Found a non-simple store.\n");
1431 Stores.push_back(St);
1432 DepChecker.addAccess(St);
1437 // Now we have two lists that hold the loads and the stores.
1438 // Next, we find the pointers that they use.
1440 // Check if we see any stores. If there are no stores, then we don't
1441 // care if the pointers are *restrict*.
1442 if (!Stores.size()) {
1443 DEBUG(dbgs() << "LAA: Found a read-only loop!\n");
1448 MemoryDepChecker::DepCandidates DependentAccesses;
1449 AccessAnalysis Accesses(TheLoop->getHeader()->getModule()->getDataLayout(),
1450 AA, LI, DependentAccesses, Preds);
1452 // Holds the analyzed pointers. We don't want to call GetUnderlyingObjects
1453 // multiple times on the same object. If the ptr is accessed twice, once
1454 // for read and once for write, it will only appear once (on the write
1455 // list). This is okay, since we are going to check for conflicts between
1456 // writes and between reads and writes, but not between reads and reads.
1459 ValueVector::iterator I, IE;
1460 for (I = Stores.begin(), IE = Stores.end(); I != IE; ++I) {
1461 StoreInst *ST = cast<StoreInst>(*I);
1462 Value* Ptr = ST->getPointerOperand();
1463 // Check for store to loop invariant address.
1464 StoreToLoopInvariantAddress |= isUniform(Ptr);
1465 // If we did *not* see this pointer before, insert it to the read-write
1466 // list. At this phase it is only a 'write' list.
1467 if (Seen.insert(Ptr).second) {
1470 MemoryLocation Loc = MemoryLocation::get(ST);
1471 // The TBAA metadata could have a control dependency on the predication
1472 // condition, so we cannot rely on it when determining whether or not we
1473 // need runtime pointer checks.
1474 if (blockNeedsPredication(ST->getParent(), TheLoop, DT))
1475 Loc.AATags.TBAA = nullptr;
1477 Accesses.addStore(Loc);
1481 if (IsAnnotatedParallel) {
1483 << "LAA: A loop annotated parallel, ignore memory dependency "
1489 for (I = Loads.begin(), IE = Loads.end(); I != IE; ++I) {
1490 LoadInst *LD = cast<LoadInst>(*I);
1491 Value* Ptr = LD->getPointerOperand();
1492 // If we did *not* see this pointer before, insert it to the
1493 // read list. If we *did* see it before, then it is already in
1494 // the read-write list. This allows us to vectorize expressions
1495 // such as A[i] += x; Because the address of A[i] is a read-write
1496 // pointer. This only works if the index of A[i] is consecutive.
1497 // If the address of i is unknown (for example A[B[i]]) then we may
1498 // read a few words, modify, and write a few words, and some of the
1499 // words may be written to the same address.
1500 bool IsReadOnlyPtr = false;
1501 if (Seen.insert(Ptr).second ||
1502 !isStridedPtr(SE, Ptr, TheLoop, Strides, Preds)) {
1504 IsReadOnlyPtr = true;
1507 MemoryLocation Loc = MemoryLocation::get(LD);
1508 // The TBAA metadata could have a control dependency on the predication
1509 // condition, so we cannot rely on it when determining whether or not we
1510 // need runtime pointer checks.
1511 if (blockNeedsPredication(LD->getParent(), TheLoop, DT))
1512 Loc.AATags.TBAA = nullptr;
1514 Accesses.addLoad(Loc, IsReadOnlyPtr);
1517 // If we write (or read-write) to a single destination and there are no
1518 // other reads in this loop then is it safe to vectorize.
1519 if (NumReadWrites == 1 && NumReads == 0) {
1520 DEBUG(dbgs() << "LAA: Found a write-only loop!\n");
1525 // Build dependence sets and check whether we need a runtime pointer bounds
1527 Accesses.buildDependenceSets();
1529 // Find pointers with computable bounds. We are going to use this information
1530 // to place a runtime bound check.
1531 bool CanDoRTIfNeeded =
1532 Accesses.canCheckPtrAtRT(PtrRtChecking, SE, TheLoop, Strides);
1533 if (!CanDoRTIfNeeded) {
1534 emitAnalysis(LoopAccessReport() << "cannot identify array bounds");
1535 DEBUG(dbgs() << "LAA: We can't vectorize because we can't find "
1536 << "the array bounds.\n");
1541 DEBUG(dbgs() << "LAA: We can perform a memory runtime check if needed.\n");
1544 if (Accesses.isDependencyCheckNeeded()) {
1545 DEBUG(dbgs() << "LAA: Checking memory dependencies\n");
1546 CanVecMem = DepChecker.areDepsSafe(
1547 DependentAccesses, Accesses.getDependenciesToCheck(), Strides);
1548 MaxSafeDepDistBytes = DepChecker.getMaxSafeDepDistBytes();
1550 if (!CanVecMem && DepChecker.shouldRetryWithRuntimeCheck()) {
1551 DEBUG(dbgs() << "LAA: Retrying with memory checks\n");
1553 // Clear the dependency checks. We assume they are not needed.
1554 Accesses.resetDepChecks(DepChecker);
1556 PtrRtChecking.reset();
1557 PtrRtChecking.Need = true;
1560 Accesses.canCheckPtrAtRT(PtrRtChecking, SE, TheLoop, Strides, true);
1562 // Check that we found the bounds for the pointer.
1563 if (!CanDoRTIfNeeded) {
1564 emitAnalysis(LoopAccessReport()
1565 << "cannot check memory dependencies at runtime");
1566 DEBUG(dbgs() << "LAA: Can't vectorize with memory checks\n");
1576 DEBUG(dbgs() << "LAA: No unsafe dependent memory operations in loop. We"
1577 << (PtrRtChecking.Need ? "" : " don't")
1578 << " need runtime memory checks.\n");
1580 emitAnalysis(LoopAccessReport() <<
1581 "unsafe dependent memory operations in loop");
1582 DEBUG(dbgs() << "LAA: unsafe dependent memory operations in loop\n");
1586 bool LoopAccessInfo::blockNeedsPredication(BasicBlock *BB, Loop *TheLoop,
1587 DominatorTree *DT) {
1588 assert(TheLoop->contains(BB) && "Unknown block used");
1590 // Blocks that do not dominate the latch need predication.
1591 BasicBlock* Latch = TheLoop->getLoopLatch();
1592 return !DT->dominates(BB, Latch);
1595 void LoopAccessInfo::emitAnalysis(LoopAccessReport &Message) {
1596 assert(!Report && "Multiple reports generated");
1600 bool LoopAccessInfo::isUniform(Value *V) const {
1601 return (SE->isLoopInvariant(SE->getSCEV(V), TheLoop));
1604 // FIXME: this function is currently a duplicate of the one in
1605 // LoopVectorize.cpp.
1606 static Instruction *getFirstInst(Instruction *FirstInst, Value *V,
1610 if (Instruction *I = dyn_cast<Instruction>(V))
1611 return I->getParent() == Loc->getParent() ? I : nullptr;
1616 /// \brief IR Values for the lower and upper bounds of a pointer evolution. We
1617 /// need to use value-handles because SCEV expansion can invalidate previously
1618 /// expanded values. Thus expansion of a pointer can invalidate the bounds for
1620 struct PointerBounds {
1621 TrackingVH<Value> Start;
1622 TrackingVH<Value> End;
1624 } // end anonymous namespace
1626 /// \brief Expand code for the lower and upper bound of the pointer group \p CG
1627 /// in \p TheLoop. \return the values for the bounds.
1628 static PointerBounds
1629 expandBounds(const RuntimePointerChecking::CheckingPtrGroup *CG, Loop *TheLoop,
1630 Instruction *Loc, SCEVExpander &Exp, ScalarEvolution *SE,
1631 const RuntimePointerChecking &PtrRtChecking) {
1632 Value *Ptr = PtrRtChecking.Pointers[CG->Members[0]].PointerValue;
1633 const SCEV *Sc = SE->getSCEV(Ptr);
1635 if (SE->isLoopInvariant(Sc, TheLoop)) {
1636 DEBUG(dbgs() << "LAA: Adding RT check for a loop invariant ptr:" << *Ptr
1640 unsigned AS = Ptr->getType()->getPointerAddressSpace();
1641 LLVMContext &Ctx = Loc->getContext();
1643 // Use this type for pointer arithmetic.
1644 Type *PtrArithTy = Type::getInt8PtrTy(Ctx, AS);
1645 Value *Start = nullptr, *End = nullptr;
1647 DEBUG(dbgs() << "LAA: Adding RT check for range:\n");
1648 Start = Exp.expandCodeFor(CG->Low, PtrArithTy, Loc);
1649 End = Exp.expandCodeFor(CG->High, PtrArithTy, Loc);
1650 DEBUG(dbgs() << "Start: " << *CG->Low << " End: " << *CG->High << "\n");
1651 return {Start, End};
1655 /// \brief Turns a collection of checks into a collection of expanded upper and
1656 /// lower bounds for both pointers in the check.
1657 static SmallVector<std::pair<PointerBounds, PointerBounds>, 4> expandBounds(
1658 const SmallVectorImpl<RuntimePointerChecking::PointerCheck> &PointerChecks,
1659 Loop *L, Instruction *Loc, ScalarEvolution *SE, SCEVExpander &Exp,
1660 const RuntimePointerChecking &PtrRtChecking) {
1661 SmallVector<std::pair<PointerBounds, PointerBounds>, 4> ChecksWithBounds;
1663 // Here we're relying on the SCEV Expander's cache to only emit code for the
1664 // same bounds once.
1666 PointerChecks.begin(), PointerChecks.end(),
1667 std::back_inserter(ChecksWithBounds),
1668 [&](const RuntimePointerChecking::PointerCheck &Check) {
1670 First = expandBounds(Check.first, L, Loc, Exp, SE, PtrRtChecking),
1671 Second = expandBounds(Check.second, L, Loc, Exp, SE, PtrRtChecking);
1672 return std::make_pair(First, Second);
1675 return ChecksWithBounds;
1678 std::pair<Instruction *, Instruction *> LoopAccessInfo::addRuntimeChecks(
1680 const SmallVectorImpl<RuntimePointerChecking::PointerCheck> &PointerChecks)
1683 SCEVExpander Exp(*SE, DL, "induction");
1684 auto ExpandedChecks =
1685 expandBounds(PointerChecks, TheLoop, Loc, SE, Exp, PtrRtChecking);
1687 LLVMContext &Ctx = Loc->getContext();
1688 Instruction *FirstInst = nullptr;
1689 IRBuilder<> ChkBuilder(Loc);
1690 // Our instructions might fold to a constant.
1691 Value *MemoryRuntimeCheck = nullptr;
1693 for (const auto &Check : ExpandedChecks) {
1694 const PointerBounds &A = Check.first, &B = Check.second;
1695 // Check if two pointers (A and B) conflict where conflict is computed as:
1696 // start(A) <= end(B) && start(B) <= end(A)
1697 unsigned AS0 = A.Start->getType()->getPointerAddressSpace();
1698 unsigned AS1 = B.Start->getType()->getPointerAddressSpace();
1700 assert((AS0 == B.End->getType()->getPointerAddressSpace()) &&
1701 (AS1 == A.End->getType()->getPointerAddressSpace()) &&
1702 "Trying to bounds check pointers with different address spaces");
1704 Type *PtrArithTy0 = Type::getInt8PtrTy(Ctx, AS0);
1705 Type *PtrArithTy1 = Type::getInt8PtrTy(Ctx, AS1);
1707 Value *Start0 = ChkBuilder.CreateBitCast(A.Start, PtrArithTy0, "bc");
1708 Value *Start1 = ChkBuilder.CreateBitCast(B.Start, PtrArithTy1, "bc");
1709 Value *End0 = ChkBuilder.CreateBitCast(A.End, PtrArithTy1, "bc");
1710 Value *End1 = ChkBuilder.CreateBitCast(B.End, PtrArithTy0, "bc");
1712 Value *Cmp0 = ChkBuilder.CreateICmpULE(Start0, End1, "bound0");
1713 FirstInst = getFirstInst(FirstInst, Cmp0, Loc);
1714 Value *Cmp1 = ChkBuilder.CreateICmpULE(Start1, End0, "bound1");
1715 FirstInst = getFirstInst(FirstInst, Cmp1, Loc);
1716 Value *IsConflict = ChkBuilder.CreateAnd(Cmp0, Cmp1, "found.conflict");
1717 FirstInst = getFirstInst(FirstInst, IsConflict, Loc);
1718 if (MemoryRuntimeCheck) {
1720 ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
1721 FirstInst = getFirstInst(FirstInst, IsConflict, Loc);
1723 MemoryRuntimeCheck = IsConflict;
1726 if (!MemoryRuntimeCheck)
1727 return std::make_pair(nullptr, nullptr);
1729 // We have to do this trickery because the IRBuilder might fold the check to a
1730 // constant expression in which case there is no Instruction anchored in a
1732 Instruction *Check = BinaryOperator::CreateAnd(MemoryRuntimeCheck,
1733 ConstantInt::getTrue(Ctx));
1734 ChkBuilder.Insert(Check, "memcheck.conflict");
1735 FirstInst = getFirstInst(FirstInst, Check, Loc);
1736 return std::make_pair(FirstInst, Check);
1739 std::pair<Instruction *, Instruction *>
1740 LoopAccessInfo::addRuntimeChecks(Instruction *Loc) const {
1741 if (!PtrRtChecking.Need)
1742 return std::make_pair(nullptr, nullptr);
1744 return addRuntimeChecks(Loc, PtrRtChecking.getChecks());
1747 LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
1748 const DataLayout &DL,
1749 const TargetLibraryInfo *TLI, AliasAnalysis *AA,
1750 DominatorTree *DT, LoopInfo *LI,
1751 const ValueToValueMap &Strides)
1752 : PtrRtChecking(SE), DepChecker(SE, L, Preds), TheLoop(L), SE(SE), DL(DL),
1753 TLI(TLI), AA(AA), DT(DT), LI(LI), NumLoads(0), NumStores(0),
1754 MaxSafeDepDistBytes(-1U), CanVecMem(false),
1755 StoreToLoopInvariantAddress(false) {
1756 if (canAnalyzeLoop())
1757 analyzeLoop(Strides);
1760 void LoopAccessInfo::print(raw_ostream &OS, unsigned Depth) const {
1762 if (PtrRtChecking.Need)
1763 OS.indent(Depth) << "Memory dependences are safe with run-time checks\n";
1765 OS.indent(Depth) << "Memory dependences are safe\n";
1769 OS.indent(Depth) << "Report: " << Report->str() << "\n";
1771 if (auto *Dependences = DepChecker.getDependences()) {
1772 OS.indent(Depth) << "Dependences:\n";
1773 for (auto &Dep : *Dependences) {
1774 Dep.print(OS, Depth + 2, DepChecker.getMemoryInstructions());
1778 OS.indent(Depth) << "Too many dependences, not recorded\n";
1780 // List the pair of accesses need run-time checks to prove independence.
1781 PtrRtChecking.print(OS, Depth);
1784 OS.indent(Depth) << "Store to invariant address was "
1785 << (StoreToLoopInvariantAddress ? "" : "not ")
1786 << "found in loop.\n";
1788 OS.indent(Depth) << "SCEV assumptions:\n";
1789 Preds.print(OS, Depth);
1792 const LoopAccessInfo &
1793 LoopAccessAnalysis::getInfo(Loop *L, const ValueToValueMap &Strides) {
1794 auto &LAI = LoopAccessInfoMap[L];
1797 assert((!LAI || LAI->NumSymbolicStrides == Strides.size()) &&
1798 "Symbolic strides changed for loop");
1802 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
1804 llvm::make_unique<LoopAccessInfo>(L, SE, DL, TLI, AA, DT, LI, Strides);
1806 LAI->NumSymbolicStrides = Strides.size();
1812 void LoopAccessAnalysis::print(raw_ostream &OS, const Module *M) const {
1813 LoopAccessAnalysis &LAA = *const_cast<LoopAccessAnalysis *>(this);
1815 ValueToValueMap NoSymbolicStrides;
1817 for (Loop *TopLevelLoop : *LI)
1818 for (Loop *L : depth_first(TopLevelLoop)) {
1819 OS.indent(2) << L->getHeader()->getName() << ":\n";
1820 auto &LAI = LAA.getInfo(L, NoSymbolicStrides);
1825 bool LoopAccessAnalysis::runOnFunction(Function &F) {
1826 SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
1827 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
1828 TLI = TLIP ? &TLIP->getTLI() : nullptr;
1829 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
1830 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
1831 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
1836 void LoopAccessAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
1837 AU.addRequired<ScalarEvolutionWrapperPass>();
1838 AU.addRequired<AAResultsWrapperPass>();
1839 AU.addRequired<DominatorTreeWrapperPass>();
1840 AU.addRequired<LoopInfoWrapperPass>();
1842 AU.setPreservesAll();
1845 char LoopAccessAnalysis::ID = 0;
1846 static const char laa_name[] = "Loop Access Analysis";
1847 #define LAA_NAME "loop-accesses"
1849 INITIALIZE_PASS_BEGIN(LoopAccessAnalysis, LAA_NAME, laa_name, false, true)
1850 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
1851 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
1852 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
1853 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
1854 INITIALIZE_PASS_END(LoopAccessAnalysis, LAA_NAME, laa_name, false, true)
1857 Pass *createLAAPass() {
1858 return new LoopAccessAnalysis();